Some heavy-duty vehicles, such as tractor-trailer trucks, utilize suspension systems that include an axle that can be selectively raised and lowered with respect to the undercarriage of the vehicle. An axle capable of being raised and lowered is commonly known in the industry as a lift axle. When the lift axle is lowered, or deployed, its wheels contact a road or other ground surface. In its lowered position, the lift axle assists the non-lift axles, known as primary axles, in bearing a portion of the vehicle or load weight. Distributing this weight across the additional axle may allow a vehicle to meet weight restrictions such as maximum weight per axle.
When the lift axle is lifted, or retracted, its wheels disengage from the road or other ground surface and no longer assist in the bearing of weight. When the load weight of the vehicle is less than the load capacity of the primary axles, the lift axle suspension may be raised to avoid extra wear on the lift axle and tires. Operating the vehicle with the lift axle suspension raised can also improve fuel economy, traction, and maneuverability. In addition, when the trailer is driven in reverse, the positive castor angle of the lift axle wheels may hinder self-steering and cause excess wear to the tires. Raising the lift axle disengages the wheels from the ground, thereby avoiding these issues.
A typical, conventional lift axle control system 900 is shown in FIG. 10. This prior art system includes a pressurized air tank 904 that provides air at the system pressure. A master valve 912 is provided between the system-pressurized air from the air tank 904 and a regulator 906. The regulator 906 is adjustable, to permit the user to set the regulated air pressure to a desired value, for example to accommodate the anticipated load and/or driving conditions. A second supply air output 905 fluidly connects the components to the system-pressurized air tank 904. As will be appreciated by persons of skill in the art, in FIG. 10 the character “S” indicates an air source connection to the associated component, the character “C” indicates a lower-pressure control or pilot air connection to the associated component, and the character “D” indicates a delivery side connection.
When the valve 912 is set to an open position, opening a fluid path to the regulator 906, the regulator 906 provides a control air pressure to a first regulated relay 901. The first regulator relay 901 inflates load bladders 918 until the load bladders 918 are at the control air pressure determined by the regulator 906. A second regulated relay 902 connects to the lift bladders 920 through an inverter valve 903, such that the lift bladders 920 are vented when the valve 912 is set to the open position. When the valve 912 is set to the closed position, closing the fluid path to the regulator 906, the first regulated relay 901 vents the load bladders 918, and the inverter valve 903 transmits a control pressure to the second regulated relay 902, such that the lift bladders 920 are inflated to the control pressure. Such prior art systems are relatively complicated, requiring two regulated relays and an inverter valve. It would be advantageous to provide a simpler pneumatic lift axle control system, that is easier and less-expensive to manufacture, and more reliable.
In addition, such prior art systems require cycling the regulator 906 on the supply side, which is hard on the regulator, and can result in premature failure of the system and/or the need for a more expensive and rugged regulator. It would therefore also be advantageous to provide a lift axle control system that does not require cycling the supply side pressure on the regulator 906 when switching between lift axle deployment or load operation, and retracted operation.